1. Field of the Invention
This invention relates to high pressure reinforced flexible hose made from synthetic thermoplastic materials. More particularly, this invention is concerned with a thermoplastic hose in which bonding between the reinforcement and an adjacent element of the hose is obtained by using a mixture of yarn and monofilament for the reinforcement.
2. Description of the Prior Art
In the manufacture of high performance hose it is important that the cover be bonded to the reinforcement and the reinforcement to the tube. This bonding contributes to the overall physical properties of the hose, improves the kink resistance, and prevents separation and draw back of the reinforcement when the hose is cut to length and fittings are attached. It is also important, to protect the reinforcement yarns from being torn or cut, that the protective cover be securely bonded to the reinforcement.
One of the more common methods that has been used to bond the reinforcement to an adjacent element of a hose is by solvating either the surface of the reinforcement or the surface of the adjacent element before they are brought into contacting relationship with each other. Solvent bonding works well in many circumstances but necessarily depends upon the availability of a solvent that is not too expensive, one that will solvate both contacting surfaces if they are made from different materials, and, from a manufacturing viewpoint, one that is not toxic and does not require special protection for production workers.
Adhesives are sometimes used as a bonding material but, in many applications, even when good initial adhesion is obtained, the bonds tend to weaken after the hose had been pressure pulsed a number of times.
Heat fusion techniques may provide good bonds, but they are seldom useful since heat softenable synthetic yarns may suffer a significant loss in tensile strength when exposed to temperatures at or near their melting points.
Another approach to the bonding problem is to make the reinforcement in the form of a comparatively open meshed braid that has open areas into which melted cover or tube material can be extruded and establish mechanical bonds when cooled. This technique may work well for locking the reinforcement in place, but an open braid markedly decreases the burst strength of a hose.
Accordingly it is an object of this invention to improve the bond strength between the reinforcement and adjacent components of a high pressure hose.
A more specific object of this invention is to improve the bond strength between the cover and the braided reinforcement of a hose without loss of burst strength.
Breifly these and other objects of this invention are achieved by reinforcing hose with a combination of yarns and monofilaments to create stereo interstices into which molten tube or cover material may be flowed to establish mechanical bonds.
The reinforcement materials which with this invention are principally concerned, are those high tenacity yarns which are difficult to bond to the other elements of a thermoplastic hose using solvent bonding techniques. These materials include thermoplastics ranging from the polyesters which are difficult to solvate to polyaramides which are essentially insoluble. In addition to synthetic polymeric reinforcing materials, the invention contemplates the use of other kinds of high tenacity reinforcing materials including yarns and monofilaments made from metal, glass and carbon fibers.
Polyester yarns are particularly useful in making hose in accordance with this invention and, for convenience, the following description is restricted to polyester yarns and monofilaments although it is not intended that the invention should be so limited.
High tenacity polyester yarns are commonly prepared by the polymeric reaction of terephthalic acid (or dimethyl phthalate) and ethylene glycol and are sold, for example, by duPont under its trademark Dacron.RTM. and ICI under its trademark Terylene.RTM.. The polyester yarns have certain properties which make them particularly suitable for use as hose reinforcement. For example, as compared with nylon, also an excellent high tenacity fiber, the polyesters are somewhat less expensive, they have higher tensile strength, they have lower elongation, and they do not absorb as much moisture. The higher strength is advantageous in that less yarn is required to obtain a given burst strength and this not only saves money but the result is a lighter, more compact, and more kink resistant hose. The low moisture absorption reduces the change in electrical conductivity which is significant in designing a nonconductive hose. The lower elongation provided by the polyester reinforcement is desirable since the hose will have a lower volumetric expansion when pressurized. Also, since a lesser weight of reinforcement is needed, the reinforcement can be packed closer to the center of the hose, thus yielding slightly higher burst pressures.
As previously mentioned, it is difficult to use solvent bonding techniques with polyesters unless somewhat obnoxious or toxic solvents are used at elevated temperatures. In the practice of this invention, mechanical bonds are formed between the polyester reinforcement and other elements of the hose by preparing the reinforcement from a mixture of yarns and monofilaments which develop stereo interstices into which the tube or cover material may be flowed to develop mechanical bonds. The reason that these interstices are developed is due to the fact that when a yarn reinforcement is pulled down, the individual fibers of the yarn are flattened out over the surface to which they are being applied (i.e., either the surface of tube or a previously applied layer of reinforcement). On the other hand, monofilaments retain much of their round cross-sectional configuration and stand away from the surface to which they are applied. For example, when the reinforcement is being applied in the form of a braid, the geometry of the intersection of the substantially circular monofilaments result in the development of small areas or pockets into which the tube or cover material may be flowed. This result takes place even though a high coverage, compact braid is applied and it is to be distinguished from the open areas that result when a comparatively loose braid is applied over a tube. Similar stereo interstices are obtained at the points of intersection of monofilaments when the reinforcement is applied as a plurality of spiral wraps of opposite hand.
Since in the practice of this invention, the reinforcement is closely packed and there is very little, if any, measurable loss in the burst strength of the hose.